Quantitative atomic force microscopy with carbon monoxide terminated tips

Noncontact atomic force microscopy (AFM) has recently progressed tremendously in achieving atomic resolution imaging through the use of small oscillation amplitudes and well-defined modification of the tip apex. In particular, it has been shown that picking up simple inorganic molecules (such as CO) by the AFM tip leads to a well-defined tip apex and to enhanced image resolution. Here, we use the same approach to study the three-dimensional intermolecular interaction potential between two molecules and focus on the implications of using molecule-modified AFM tips for microscopy and force spectroscopy experiments. The flexibility of the CO at the tip apex complicates the measurement of the intermolecular interaction energy between two CO molecules. Our work establishes the physical limits of measuring intermolecular interactions with scanning probes.     

Ageing of a microscopic sliding gold contact at low temperatures

Nanometer-scale friction measurements on a Au(111) surface have been performed at temperatures between 30 and 300?K by means of atomic force microscopy. Stable stick slip with atomic periodicity is observed at all temperatures, showing only weak dependence on temperature between 300 and 170?K. Below 170?K, friction increases with time and a distortion of the stick-slip characteristic is observed. Low friction and periodic stick slip can be reestablished by pulling the tip out of contact and subsequently restoring the contact. A comparison with molecular dynamics simulations indicates that plastic deformation within a growing gold junction leads to the observed frictional behavior at low temperatures. The regular stick slip with atomic periodicity observed at room temperature is the result of a dynamic equilibrium shape of the contact, as microscopic wear damage is observed to heal in the sliding contact.     

Regular modes in rotating stars

Despite more and more observational data, stellar acoustic oscillation modes are not well understood as soon as rotation cannot be treated perturbatively. In a way similar to semiclassical theory in quantum physics, we use acoustic ray dynamics to build an asymptotic theory for the subset of regular modes which are the easiest to observe and identify. Comparisons with 2D numerical simulations of oscillations in polytropic stars show that both the frequency and amplitude distributions of these modes can accurately be described by an asymptotic theory for almost all rotation rates. The spectra are mainly characterized by two quantum numbers; their extraction from observed spectra should enable one to obtain information about stellar interiors.     

Higher-order modulation instability in nonlinear fiber optics

We report theoretical, numerical, and experimental studies of higher-order modulation instability in the focusing nonlinear Schr?dinger equation. This higher-order instability arises from the nonlinear superposition of elementary instabilities, associated with initial single breather evolution followed by a regime of complex, yet deterministic, pulse splitting. We analytically describe the process using the Darboux transformation and compare with experiments in optical fiber. We show how a suitably low frequency modulation on a continuous wave field induces higher-order modulation instability splitting with the pulse characteristics at different phases of evolution related by a simple scaling relationship. We anticipate that similar processes are likely to be observed in many other systems including plasmas, Bose-Einstein condensates, and deep water waves.     

Faithful entanglement swapping based on sum-frequency generation

We show that an entanglement swapping operation performed with spontaneous parametric down-conversion can be made faithful without postselection using sum-frequency generation. This invites us to revisit the sum-frequency process and from a proof-of-principle experiment, we demonstrate that it provides a realistic solution for nonlinear optics at the single-photon level. This opens the way to attractive alternatives to six-photon protocols based on linear optics used, e.g., for the heralded creation of maximally entangled pairs or for device-independent quantum key distribution.     

Ultrasmall subwavelength nanorod plasmonic cavity

We propose an ultrasmall plasmonic cavity consisting of a high-index/low-index dielectric nanorod covered with silver. Full three-dimensional subwavelength confinement of the surface-plasmon polaritons was achieved at the high-index dielectric-silver interface without propagating to the low-index dielectric-silver interface. The numerical simulations showed that the plasmonic mode excited in this cavity has a deep subwavelength mode volume of 0.0038(λ/2n)(3) and a quality factor of 1500 at 40 K, and consequently a large Purcell factor of ～2×10(5). Therefore, this plasmonic cavity is expected to be useful for the demonstration of high-efficiency single photon sources or low-threshold lasers in an ultracompact nanophotonic circuit.     

Coherent beam combination of narrow-linewidth 1.5 μm fiber amplifiers in a long-pulse regime

We report what we believe to be the first experimental demonstration of coherent beam combining of two fiber amplifiers in a 100 ns pulse regime using a signal leak between the pulses. Pulses of ～100 W stimulated-Brillouin-scattering limited peak power are combined with 95% efficiency, a residual phase error of λ/27, and no significant beam quality degradation.     

CnH2nCl+ ion formation in electron impact MS conditions: a theoretical study

The mass spectral fragmentation of different 1-chloroalkanes (of the 1-chlorohexane-1-chlorooctadecane series) has been investigated, quantifying the relative abundance of the fragment ions. The base peak is dominantly at m/z 91, 93 in each investigated case, although with the increasing chain length, its contribution to the total ion current exhibits some reduction. Among the possible fragmentation products, the five-membered chloronium containing ring is the most stable as measured by an isodesmic reaction, although the six-and seven-membered rings exhibit only slightly reduced stability. The most stable structure of the 1-chlorohexane radical cation has a hydrogen bonded structure with the involvement of chlorine and the H_C(δ), pre-forming the five-membered cationic ring. Accordingly, among the reactions leading to alkyl (or H) radical and a chloronium containing ring, this transition structure has the lowest energy, providing explanation for the experimental observations.     

Catalytic Zinc Complexes for Phosphate Diester Hydrolysis

Creating efficient artificial catalysts that can compete with biocatalysis has been an enduring challenge which has yet to be met. Reported herein is the synthesis and characterization of a series of zinc complexes designed to catalyze the hydrolysis of phosphate diesters. By introducing a hydrated aldehyde into the ligand we achieve turnover for DNA‐like substrates which, combined with ligand methylation, increases reactivity by two orders of magnitude. In contrast to current orthodoxy and mechanistic explanations, we propose a mechanism where the nucleophile is not coordinated to the metal ion, but involves a tautomer with a more effective Lewis acid and more reactive nucleophile. This data suggests a new strategy for creating more efficient metal ion based catalysts, and highlights a possible mode of action for metalloenzymes.     

Modular Construction of Dynamic Nucleodendrimers

Isoguanosine‐containing dendritic small molecules self‐assemble into decameric nucleodendrimers as observed by 1D NMR spectroscopy, 2D DOSY, and mass spectrometry. In particular, apolar building blocks readily form pentameric structures in acetonitrile while the presence of alkali metals promotes the formation of stable decameric assemblies with a preference for cesium ions. Remarkably, co‐incubation of guanosine and isoguanosine‐containing nucleodendrons results in the formation of decameric structures in absence of added salts. Further analysis of the mixture indicated that guanosine derivatives facilitate the formation, but are not involved in decameric structures; a process reminiscent of molecular crowding. This molecular system provides a powerful canvas for the rapid and modular assembly of polyfunctional dendritic macromolecules.